Laser Produced Plasma EUV Light Source

ABSTRACT

A device is disclosed herein which may include a plasma generating system comprising a source of target material droplets and a laser producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation, wherein the droplet source comprises a fluid exiting an orifice and a sub-system producing a disturbance in the fluid which generates droplets having differing initial velocities causing the spacing between at least some adjacent droplets to decrease as the droplets travel to the irradiation region.

The present application is a continuation-in-part application of co-pending U.S. patent application Ser. No. 11/827,803, filed on Jul. 13, 2007, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE HAVING A DROPLET STREAM PRODUCED USING A MODULATED DISTURBANCE WAVE, Attorney Docket Number 2007-0030-01; the entire contents of which is hereby incorporated by reference herein.

The present application is related to co-pending U.S. patent application Ser. No. 11/358,988 filed on Feb. 21, 2006, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, Attorney Docket Number 2005-0085-01; co-pending U.S. patent application Ser. No. 11/067,124 filed on Feb. 25, 2005, entitled METHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, Attorney Docket Number 2004-0008-01; co-pending U.S. patent application Ser. No. 11/174,443 filed on Jun. 29, 2005, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, Attorney Docket Number 2005-0003-01; co-pending U.S. SOURCE MATERIAL DISPENSER FOR EUV LIGHT SOURCE, Attorney Docket Number 2005-0102-01, co-pending U.S. patent application Ser. No. 11/358,992 filed on Feb. 21, 2006, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, Attorney Docket Number 2005-0081-01; co-pending U.S. patent application Ser. No. 11/174,299 filed on Jun. 29, 2005, and entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM, Attorney Docket Number 2005-0044-01; co-pending U.S. patent application Ser. No. 11/406,216 filed on Apr. 17, 2006, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0003-01; co-pending U.S. patent application Ser. No. 11/580,414 filed on Oct. 13, 2006, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0025-01; and co-pending U.S. patent application Ser. No. 11/644,153 filed on Dec. 22, 2006, entitled, LASER PRODUCED PLASMA EUV IGHT SOURCE, Attorney Docket Number 2006-006-01; co-pending U.S. patent application Ser. No. 11/505,177 filed on Aug. 16, 2006, entitled EUV OPTICS, Attorney Docket Number 2006-0027-01; co-pending U.S. patent application Ser. No. 11/452,558 filed on Jun. 14, 2006, entitled DRIVE LASER FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0001-01; co-pending U.S. Pat. No. 6,928,093, issued to Webb, et al., on Aug. 9, 2005, entitled LONG DELAY AND HIGH TIS PULSE STRETCHER, U.S. application Ser. No. 11/394,512, Attorney Docket Number 2004-0144-01, filed on Mar. 31, 2006, entitled CONFOCAL PULSE STRETCHER; U.S. application Ser. No. 11/138,001 (Attorney Docket Number 2004-0128-01) filed on May 26, 2005, entitled SYSTEMS AND METHODS FOR IMPLEMENTING AN INTERACTION BETWEEN A LASER SHAPED AS A LINE BEAM AND A FILM DEPOSITED ON A SUBSTRATE; and U.S. application Ser. No. 10/141,216, filed on May 7, 2002, now U.S. Pat. No. 6,693,939, entitled, LASER LITHOGRAPHY LIGHT SOURCE WITH BEAM DELIVERY; U.S. Pat. No. 6,625,191 issued to Knowles, et al., on Sep. 23, 2003, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM; U.S. application Ser. No. 10/012,002, Attorney Docket Number 2001-0090-01, U.S. Pat. No. 6,549,551 issued to Ness, et al., on Apr. 15, 2003, entitled INJECTION SEEDED LASER WITH PRECISE TIMING CONTROL, U.S. application Ser. No. 09/848,043, Attorney Docket Number 2001-0020-01; and U.S. Pat. No. 6,567,450 issued to Myers, et al., on May 20, 2003, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, U.S. application Ser. No. 09/943,343, Attorney Docket Number 2001-0084-01; co-pending U.S. patent application Ser. No. 11/509,925 filed on Aug. 25, 2006, entitled SOURCE MATERIAL COLLECTION UNIT FOR A LASER PRODUCED PLASMA BUY LIGHT SOURCE, Attorney Docket Number 2005-0086-01; the entire contents of each of which are hereby incorporated by reference herein.

FIELD

The present disclosure relates to extreme ultraviolet (“EUV”) light sources that provide EUV light from a plasma that is created from a target material and collected and directed to an intermediate region for utilization outside of the EUV light source chamber, e.g., by a lithography scanner/stepper.

BACKGROUND

Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.

Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material having the required line-emitting element, with a laser beam.

One particular LPP technique involves irradiating a target material droplet with one or more pre-pulse(s) followed by a main pulse. In this regard, CO₂ lasers may present certain advantages as a drive laser producing “main” pulses in an LPP process. This may be especially true for certain target materials such as molten tin droplets. For example, one advantage may include the ability to produce a relatively high conversion efficiency e.g., the ratio of output EUV in-band power to drive laser input power.

In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV, The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a distance from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. In more quantitative terms, one arrangement that is currently being developed with the goal of producing about 100 W at the intermediate location contemplates the use of a pulsed, focused 10-12 kW CO₂ drive laser which is synchronized with a droplet generator to sequentially irradiate about 40,000-100,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of droplets at a relatively high repetition rate (e.g., 40-100 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position (i.e. with very small “jitter”) over relatively long periods of time.

For a typical LPP setup, target material droplets are generated and then travel within a vacuum chamber to an irradiation site where they are irradiated, e.g. by a focused laser beam. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber (e.g., debris) that can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements. These debris can include high-energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of source material. For this reason, it is often desirable to use so-called “mass limited” droplets of source material to reduce or eliminate the formation of debris. The use of “mass limited” droplets may also result in a reduction in source material consumption. Techniques to achieve a mass-limited droplet may involve diluting the source material and/or using relatively small droplets. For example, the use of droplets as small as 10-50 μm is currently contemplated.

In addition to their effect on optical elements in the vacuum chamber, the plasma by-products may also adversely affect the droplet(s) approaching the irradiation site (i.e., subsequent droplets in the droplet stream). In some cases, interactions between droplets and the plasma by-products may result in a lower EUV output for these droplets. In this regard, U.S. Pat. No. 6,855,943 (hereinafter the '943 patent) which issued to Shields on Feb. 15, 2005, and is entitled “DROPLET TARGET DELIVERY METHOD FOR HIGH PULSE-RATE LASER-PLASMA EXTREME ULTRAVIOLET LIGHT SOURCE” discloses a technique in which only some of the droplets in a droplet stream, e.g., every third droplet, is irradiated to produce a pulsed EUV light output. As disclosed in the '943 patent, the nonparticipating droplets (so-called buffer droplets) advantageously shield the next participating droplet from the effects of the plasma generated at the irradiation site. However, the use of buffer droplets may increase source material consumption and/or vacuum chamber contamination and/or may require droplet generation at a frequency much higher (e.g., by a factor of two or more) than required without the use of buffer droplets. On the other hand, if the spacing between droplets can be increased, the use of buffer droplets may be reduced or eliminated. Thus, droplet size, spacing and timing consistency (i.e., jitter) are among the factors to be considered when designing a droplet generator for an LPP EUV light source.

One technique for generating droplets involves melting a target material, e.g., tin, and then forcing it under high pressure through a relative small diameter orifice, e.g. 0.5-30 μm. Under most conditions, naturally occurring instabilities, e.g. noise, in the stream exiting the orifice may cause the stream to break-up into droplets. In order to synchronize the droplets with optical pulses of the LPP drive laser, a repetitive disturbance with an amplitude exceeding that of the random noise may be applied to the continuous stream. By applying a disturbance at the same frequency (or its higher harmonics) as the repetition rate of the pulsed laser, the droplets can be synchronized with the laser pulses. In the past, the disturbance has typically been applied to the stream by driving an electro-actuatable element (such as a piezoelectric material) with a waveform of a single frequency such as a sinusoidal waveform or its equivalent.

As used herein, the term “electro-actuatable element” and its derivatives, means a material or structure which undergoes a dimensional change when subjected to a voltage, electric field, magnetic field, or combinations thereof and includes, but is not limited to, piezoelectric materials, electrostrictive materials and magnetostrictive materials.

In general, for the application of single frequency, non-modulated waveform disturbances such as a sinusoidal waveform, the spacing between droplets increases as the disturbance frequency decreases (i.e., holding other factors such as pressure and orifice diameter constant). However, as disclosed in “prop Formation From A Vibrating Orifice Generator Driven By Modulated Electrical Signals” (G. Brenn and U. Lackermeier, Phys. Fluids 9, 3658 (1997), the contents of which are incorporated by reference herein), for disturbance frequencies below about 0.3 υ(πd), where υ is the stream velocity and d is the diameter of the continuous liquid stream, more than one droplet may be generated for each disturbance period. Thus, for a 10 μm liquid jet at a stream velocity of about 50 m/s, the calculated minimum frequency below which more than one droplet per period may be produced is about 480 kHz (note: it is currently envisioned that a droplet repetition rate of 40-100 kHz and velocities of about 30-100 m/s may be desirable for LPP EUV processes). The net result is that for the application of single frequency, non-modulated waveform disturbances, the spacing between droplets is fundamentally limited and cannot exceed approximately 3.337 πd. As indicated above, it may be desirable to supply a sufficient distance between adjacent droplets in the droplet stream to reduce/eliminate the effect of the debris from the plasma on approaching droplet(s). Moreover, because the limitation on spacing is proportional to stream diameter, and as a consequence droplet size, this limitation can be particularly severe in applications such as LPP EUV light sources where relatively small, mass-limited, droplets are desirable (see discussion above).

With the above in mind, Applicants disclose a laser produced plasma, EUV light source having a droplet stream produced using a modulated disturbance waveform, and corresponding methods of use.

SUMMARY

In a first aspect, a device may include a plasma generating system comprising a source of target material droplets and a laser producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation, wherein the droplet source comprises a fluid exiting an orifice and a sub-system producing a disturbance in the fluid which generates droplets having differing initial velocities causing the spacing between at least some adjacent droplets to decrease as the droplets travel to the irradiation region.

In a particular implementation, at least some adjacent droplets may coalesce prior to reaching the irradiation region.

In one embodiment, the sub-system may produce a droplet pattern wherein droplet doublets reach the irradiation site with each droplet in the droplet doublet to being irradiated to produce plasma.

In one particular embodiment, at least one droplet in the droplet doublet may have a diameter, d, approaching the irradiation site, and center-to-center droplet spacing within each doublet, D, with d≦D≦4d.

In a particular embodiment, the disturbance may comprise a frequency modulated disturbance waveform.

In another particular embodiment, the disturbance may comprise an amplitude modulated disturbance waveform.

In one arrangement of the device, the sub-system may produce a series of pulsed disturbances, with each pulsed disturbance having at least one of a sufficiently short rise-time and sufficiently short fall-time to generate a fundamental frequency and at least one harmonic of the fundamental frequency.

In another aspect, a device may comprise a plasma generating system comprising a source of target material droplets and a laser producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation, wherein the droplet source comprises a fluid exiting an orifice and a sub-system having an electro-actuatable element producing a disturbance in the fluid, the electro-actuatable element driven by a waveform having at least one of a sufficiently short rise-time and a sufficiently short fall-time to generate a fundamental frequency within an operable response range of the electro-actuatable element and at least one harmonic of the fundamental frequency.

In one embodiment of this aspect, the waveform may be selected from the group of waveforms consisting of a square wave, rectangular wave and peaked-non-sinusoidal wave.

In a particular embodiment of this aspect, the peaked-non-sinusoidal wave may be selected from the group of waveforms consisting of a fast pulse waveform, a fast ramp waveform and a sine function waveform.

In one particular embodiment of this aspect, the at least one harmonic may be an odd harmonic of the fundamental frequency.

In one embodiment of this aspect, the at least one harmonic may comprise at to least one even harmonic of the fundamental frequency and at least one odd harmonic of the fundamental frequency.

In a particular embodiment of this aspect, the waveform may generate droplets having differing initial velocities causing the spacing between at least some adjacent droplets to decrease as the droplets travel to the irradiation region.

In one particular embodiment of this aspect, at least some adjacent droplets coalesce prior to reaching the irradiation region.

In a particular embodiment of this aspect, each pulse may be generated by changing an on-off switch state.

In another aspect, a method may include the acts/steps of flowing a fluid toward an orifice; disturbing the fluid to generate a first droplet and a second droplet, the first droplet having a different initial velocity than the second droplet causing the spacing between the first and second droplets to decrease as the droplets travel to an irradiation region; and irradiating droplet material at the irradiation region to form a plasma.

In one implementation of this aspect, the first and second droplets coalesce prior to reaching the irradiation region.

In a particular implementation of this aspect, the disturbance may comprise a waveform selected from the group of modulated waveforms consisting of a frequency modulated disturbance waveform and an amplitude modulated disturbance waveform.

In one particular implementation of this aspect, the step of disturbing the fluid may comprise producing a series of pulsed disturbances, with each pulsed disturbance having at least on of a sufficiently short rise-time and a sufficiently short fall-time to generate a fundamental frequency and at least one sub-harmonic of the fundamental frequency.

In a particular implementation, the pulsed disturbance may be selected from the group of disturbance waveforms consisting of a square wave, rectangular wave, and peaked non-sinusoidal wave.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified, schematic view of a laser produced plasma EUV light source;

FIG. 2 shows a schematic a simplified droplet source;

FIGS. 2A-2D illustrate several different techniques for coupling an electro-actuable element with a fluid to create a disturbance in a stream exiting an orifice;

FIG. 3 (Prior Art) illustrates the pattern of droplets resulting from a single frequency, non-modulated disturbance waveform;

FIG. 4 illustrates the pattern of droplets resulting from an amplitude modulated disturbance waveform;

FIG. 5 illustrates the pattern of droplets resulting from a frequency modulated disturbance waveform;

FIG. 6 shows photographs of tin droplets obtained for a single frequency, non-modulated waveform disturbance and several frequency modulated waveform disturbances;

FIG. 7 illustrates a droplet pattern achievable using a modulated waveform disturbance in which droplet pairs reach the irradiation region allowing one droplet to shield subsequent droplet pairs from plasma debris;

FIG. 8 illustrates a droplet pattern achievable using a modulated waveform disturbance in which droplet pairs reach the irradiation region with a first droplet reflecting light into a self-directing laser system to initiate a discharge which irradiates the second droplet to produce an EUV emitting plasma;

FIG. 8A illustrates a droplet pattern in which droplet doublets reach the irradiation site with each droplet in the droplet doublet being irradiated to produce plasma;

FIG. 9 shows a representation of a square wave as a superposition of odd harmonics of a sine wave signal;

FIG. 10 shows images of droplets obtained with a square wave modulation at 30 kHz taken at ˜40 mm from the output orifice;

FIG. 11 shows images of droplets obtained with a square wave modulation at 30 kHz taken at ˜120 mm from the output orifice;

FIGS. 12A-D show experimental results for a rectangular wave (FIG. 12A) modulation including a frequency spectrum (FIG. 12B) for a rectanular wave; an image of droplets taken at 20 mm from the output orifice (FIG. 12C) and an image of coalesced droplets taken at 450 mm from the output orifice (FIG. 12D);

FIGS. 13A-D show experimental results for fast pulses (FIG. 13A) modulation including a frequency spectrum (FIG. 13B) for a fast pulse; an image of droplets taken at 20 mm from the output orifice (FIG. 13C) and an image of coalesced droplets taken at 450 mm from the output orifice (FIG. 13D);

FIGS. 14A-D show experimental results for fast ramp wave (FIG. 14A) modulation including a frequency spectrum (FIG. 14B) for a fast ramp wave; an image of droplets taken at 20 mm from the output orifice (FIG. 14C) and an image of coalesced droplets taken at 450 mm from the output orifice (FIG. 14D); and

FIGS. 15A-D show experimental results for a sine function wave (FIG. 15A) modulation including a frequency spectrum (FIG. 15B) for a sine function wave; an image of droplets taken at 20 mm from the output orifice (FIG. 15C) and an image of coalesced droplets taken at 450 mm from the output orifice (FIG. 15D).

DETAILED DESCRIPTION

With initial reference to FIG. 1, there is shown a schematic view of an EUV light source, e.g., a laser-produced-plasma, EUV light source 20 according to one aspect of an embodiment. As shown in FIG. 1, and described in further detail below, the LPP light source 20 may include a system 22 for generating a train of light pulses and delivering the light pulses into a chamber 26. As detailed below, each light pulse may travel along a beam path from the system 22 and into the chamber 26 to illuminate a respective target droplet at an irradiation region 28.

Suitable lasers for use as the system 22 shown in FIG. 1 may include a pulsed laser device, e.g., a pulsed gas discharge CO₂ laser device producing radiation at 9.3μm or 10.6 μm, e.g., with DC or RF excitation, operating at relatively high power, e.g., 10 kW or higher and high pulse repetition rate, e.g., 50 kHz or more. In one particular implementation, the laser may be an axial-flow RF-pumped CO₂ laser having a MOPA configuration with multiple stages of amplification and having a seed pulse that is initiated by a Q-switched Master Oscillator (MO) with low energy and high repetition rate, e.g., capable of 100 kHz operation. From the MO, the laser pulse may then be amplified, shaped, and/or focused before entering the LPP chamber. Continuously pumped CO₂ amplifiers may be used for the system 22. For example, a suitable CO₂ laser device having an oscillator and three amplifiers (O-PA1-PA2-PA3 configuration) is disclosed in co-pending U.S. patent application Ser. No. 11/174,299 filed on Jun. 29, 2005, entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM, Attorney Docket Number 2005-0044-01, the entire contents of which have been previously incorporated by reference herein. Alternatively, the laser may be configured as a so-called “self-targeting” laser system in which the droplet serves as one mirror of the optical cavity. In some “self-targeting” arrangements, a master oscillator may not be required. Self-targeting laser systems are disclosed and claimed in co-pending U.S. patent application Ser. No. 11/580,414, filed on Oct. 13, 2006 entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0025-01, the entire contents of which have been previously incorporated by reference herein.

Depending on the application, other types of lasers may also be suitable, e.g., an excimer or molecular fluorine laser operating at high power and high pulse repetition rate. Examples include, a solid state laser, e.g., having a fiber or disk shaped active media, an excimer laser having one or more chambers, e.g., an oscillator chamber and one or more amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator/power oscillator (MOPO) arrangement, a power oscillator/power amplifier (POPA) arrangement, or a solid state laser that seeds one or more excimer or molecular fluorine amplifier or oscillator chambers, may be suitable. Other designs are possible.

As further shown in FIG. 1, the EUV light source 20 may also include a target material delivery system 24, e.g., delivering droplets of a target material into the interior of a chamber 26 to the irradiation region 28 where the droplets will interact with one or more light pulses, e.g., zero, one or more pre-pulses and thereafter one or more main pulses, to ultimately produce a plasma and generate an EUV emission. The target material may include, but is not necessarily limited to, a material that includes tin, lithium, xenon or combinations thereof. The EUV emitting element, e.g., tin, lithium, xenon, etc., may be in the form of liquid droplets and/or solid particles contained within liquid droplets. For example, the element tin may be used as pure tin, as a tin compound, e.g., SnBr₄, SnBr₂, SnH₄, as a tin alloy, e.g., tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or a combination thereof. Depending on the material used, the target material may be presented to the irradiation region 28 at various temperatures including room temperature or near room temperature (e.g., tin alloys, SnBr₄) at an elevated temperature, (e.g., pure tin) or at temperatures below room temperature, (e.g., SnH₄), and in some cases, can be relatively volatile, e.g., SnBr₄. More details concerning the use of these materials in an LPP EUV source is provided in co-pending U.S. patent application Ser. No. 11/406,216, filed on Apr. 17, 2006, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0003-01, the contents of which have been previously incorporated by reference herein.

Continuing with FIG. 1, the EUV light source 20 may also include an optic 30, e.g., a collector mirror in the form of a truncated ellipsoid having, e.g., a graded multi-layer coating with alternating layers of Molybdenum and Silicon. FIG. 1 shows that the optic 30 may be formed with an aperture to allow the light pulses generated by the system 22 to pass through and reach the irradiation region 28. As shown, the optic 30 may be, e.g., an ellipsoidal mirror that has a first focus within or near the irradiation region 28, and a second focus at a so-called intermediate region 40, where the EUV light may be output from the EUV light source 20 and input to a device utilizing EUV light, e.g., an integrated circuit lithography tool (not shown). It is to be appreciated that other optics may be used in place of the ellipsoidal mirror for collecting and directing light to an intermediate location for subsequent delivery to a device utilizing EUV light, for example, the optic may be parabolic or may be configured to deliver a beam having a ring-shaped cross-section to an intermediate location, see e.g. co-pending U.S. patent application Ser. No. 11/505,177 filed on Aug. 16, 2006, entitled EUV OPTICS, Attorney Docket Number 2006-0027-01, the contents of which are hereby incorporated by reference.

Continuing with reference to FIG. 1, the EUV light source 20 may also include an EUV controller 60, which may also include a firing control system 65 for triggering one or more lamps and/or laser devices in the system 22 to thereby generate light pulses for delivery into the chamber 26. The EUV light source 20 may also include a droplet position detection system which may include one or more droplet imagers 70 that provide an output indicative of the position of one or more droplets, e.g., relative to the irradiation region 28. The imager(s) 70 may provide this output to a droplet position detection feedback system 62, which can, e.g., compute a droplet position and trajectory, from which a droplet error can be computed, e.g., on a droplet-by-droplet basis, or on average. The droplet error may then be provided as an input to the controller 60, which can, for example, provide a position, direction and/or timing correction signal to the system 22 to control a source timing circuit and/or to control a beam position and shaping system, e.g., to change the location and/or focal power of the light pulses being delivered to the irradiation region 28 in the chamber 26.

The EUV light source 20 may include one or more EUV metrology instruments for measuring various properties of the EUV light generated by the source 20. These properties may include, for example, intensity (e.g., total intensity or intensity within a particular spectral band), spectral bandwidth, polarization, beam position, pointing, etc. For the EUV light source 20, the instrument(s) may be configured to operate while the downstream tool, e.g., photolithography scanner, is-on-line, e.g., by sampling a portion of the EUV output, e.g., using a pickoff mirror or sampling “uncollected” EUV light, and/or may operate while the downstream tool, e.g., photolithography scanner, is off-line, for example, by measuring the entire EUV output of the EUV light source 20.

As further shown in FIG. 1, the EUV light source 20 may include a droplet control system 90, operable in response to a signal (which, in some implementations may include the droplet error described above, or some quantity derived therefrom) from the controller 60, to e.g., modify the release point of the target material from a droplet source 92 and/or modify droplet formation timing, to correct for errors in the droplets arriving at the desired irradiation region 28 and/or synchronize the generation of droplets with the pulsed laser system 22.

FIG. 2 illustrates the components of a simplified droplet source 92 in schematic format. As shown there, the droplet source 92 may include a reservoir 94 holding a fluid, e.g. molten tin, under pressure. Also shown, the reservoir 94 may be formed with an orifice 98 allowing the pressurized fluid 96 to flow through the orifice establishing a continuous stream 100 which subsequently breaks into a plurality of droplets 102 a, b.

Continuing with FIG. 2, the droplet source 92 shown further includes a sub-system producing a disturbance in the fluid having an electro-actuatable element 104 that is operably coupled with the fluid 96 and a signal generator 106 driving the electro-actuatable element 104. FIGS. 2A-2D show various ways in which one or more electro-actuatable elements may be operably coupled with the fluid to create droplets. Beginning with FIG. 2A, an arrangement is shown in which the fluid is forced to flow from a reservoir 108 under pressure through a tube 110, e.g., capillary tube, having an inside diameter between about 0.5-0.8 mm, and a length of about 10 to 50 mm, creating a continuous stream 112 exiting an orifice 114 of the tube 110 which subsequently breaks up into droplets 116 a,b. As shown, an electro-actuatable element 118 may be coupled to the tube, For example, an electro-actuatable element may be coupled to the tube 110 to deflect the tube 110 and disturb the stream 112. FIG. 2B shows a similar arrangement having a reservoir 120, tube 122 and a pair of electro-actuatable elements 124, 126, each coupled to the tube 122 to deflect the tube 122 at a respective frequency, FIG. 2C shows another variation in which a plate 128 is positioned in a reservoir 130 moveable to force fluid through an orifice 132 to create a stream 134 which breaks into droplets 136 a,b. As shown, a force may be applied to the plate 128 and one or more electro-actuatable elements 138 may be coupled to the plate to disturb the stream 134. It is to be appreciated that a capillary tube may be used with the embodiment shown in FIG. 2C, FIG. 2D shows another variation, in which a fluid is forced to flow from a reservoir 140 under pressure through a tube 142 creating a continuous stream 144, exiting an orifice 146 of the tube 142, which subsequently breaks-up into droplets 148 a,b. As shown, an electro-actuatable element 150, e.g., having a ring-like shape, may be positioned around the tube 142. When driven, the electro-actuatable element 142 may selectively squeeze and/or un-squeeze the tube 142 to disturb the stream 144. It is to be appreciated that two or more electro-actuatable elements may be employed to selectively squeeze the tube 142 at respective frequencies.

More details regarding various droplet dispenser configurations and their relative advantages may be found in co-pending U.S. patent application Ser. No. 11/358,988, filed on Feb. 21, 2006, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, Attorney Docket Number 2005-0085-01; co-pending U.S. patent application Ser. No. 11/067,124 filed on Feb. 25, 2005, entitled METHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, Attorney Docket Number 2004-0008-01; and co-pending U.S. patent application Ser. No. 11/174,443 filed on Jun. 29, 2005, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, Attorney Docket Number 2005-0003-01; the contents of each of which are hereby incorporated by reference.

FIG. 3 (Prior Art) illustrates the pattern of droplets 200 resulting from a single frequency, sine wave disturbance waveform 202 (for disturbance frequencies above about 0.3 υ/(πd). It can be seen that each period of the disturbance waveform produces a droplet. FIG. 3 also illustrates that the droplets do not coalesce together, but rather, each droplet is established with the same initial velocity.

FIG. 4 illustrates the pattern of droplets 300 initially resulting from an amplitude modulated disturbance waveform 302, which however is unlike the disturbance waveform 202 described above, in that it is not limited to disturbance frequencies above about 0.3 υ(πd)). It can be seen that the amplitude modulated waveform disturbance 302 includes two characteristic frequencies, a relatively large frequency, e.g., carrier frequency, corresponding to wavelength π_(c), and a smaller frequency, e.g., modulation frequency, corresponding to wavelength, π_(m). For the specific disturbance waveform example shown in FIG. 4, the modulation frequency is a carrier frequency subharmonic, and in particular, the modulation frequency is a third of the carrier frequency. With this waveform, FIG. 4 illustrates that each period of the disturbance waveform corresponding to the carrier wavelength, λ_(c) produces a droplet. FIG. 4 also illustrates that the droplets coalesce together, resulting in a stream of larger droplets 304, with one larger droplet for each period of the disturbance waveform corresponding to the modulation wavelength, λ_(m). Arrows 306 a,b show the initial relative velocity components that are imparted on the droplets by the modulated waveform disturbance 302, and are responsible for the droplet coalescence.

FIG. 5 illustrates the pattern of droplets 400 initially resulting from a frequency modulated disturbance waveform 402, which, like the disturbance waveform 302 described above, is not limited to disturbance frequencies above about 0.30υ/(πd). It can be seen that the frequency modulated waveform disturbance 402 includes two characteristic frequencies, a relatively large frequency, e.g. carrier frequency, corresponding to wavelength λ_(c), and a smaller frequency, e.g. modulation frequency, corresponding to wavelength, λ_(m). For the specific disturbance waveform example shown in FIG. 5, the modulation frequency is a carrier frequency harmonic, and in particular, the modulation frequency is a third of the carrier frequency. With this waveform, FIG. 5 illustrates that each period of the disturbance waveform corresponding to the carrier wavelength, λ_(c) produces a droplet. FIG. 5 also illustrates that the droplets coalesce together, resulting in a stream of larger droplets 404, with one larger droplet for each period of the disturbance waveform corresponding to the modulation wavelength, λ_(m). Like the amplitude modulated disturbance (i.e., FIG. 4), initial relative velocity components are imparted on the droplets by the frequency modulated waveform disturbance 402, and are responsible for the droplet coalescence.

Although FIGS. 4 and 5 show and discuss embodiments having two characteristic frequencies, with FIG. 4 illustrating an amplitude modulated disturbance having two characteristic frequencies, and FIG. 5 illustrating a frequency modulated disturbance having two frequencies, it is to be appreciated that more than two characteristic frequencies may be employed and that the modulation may be either angular modulation (i.e., frequency or phase modulation), amplitude modulation, or combinations thereof.

FIG. 6 shows photographs of tin droplets obtained using an apparatus similar to FIG. 2D with an orifice diameter of about 70 μm, stream velocity of ˜30 m/s, for a single frequency, non-modulated waveform disturbance having a frequency of 100 kHz (top photo); a frequency modulated waveform disturbance having a carrier frequency of 100 kHz and a modulating frequency of 10 kHz of a relatively strong modulation depth (second from top photo); a frequency modulated waveform disturbance having a carrier frequency of 100 kHz and a modulating frequency of 10 kHz of a relatively weak modulation depth (third from top photo); a frequency modulated waveform disturbance having a carrier frequency of 100 kHz and a modulating frequency of 15 kHz (fourth from top photo), a frequency modulated waveform disturbance having a carrier frequency of 100 kHz and a modulating frequency of 20 kHz (bottom photo).

These photographs indicate that tin droplets having a diameter of about 265 μm can be produced that are spaced apart by about 3.14 mm, a spacing which cannot be realized at this droplet size and repetition rate using a single frequency, non-modulated waveform disturbance.

Measurements indicated a timing jitter of about 0.14% of a modulation period which is substantially less than the jitter observed under similar conditions using a single frequency, non-modulated waveform disturbance. This effect is achieved because the individual droplet instabilities are averaged over a number of coalescing droplets.

FIG. 7 shows a droplet pattern 600 produced using a modulated, e.g., multiple frequency, disturbance waveform (see also FIG. 6, fourth photo from top). Also shown, droplet pairs are formed at a selected distance from orifice 604. As shown, this droplet pattern in which droplet pairs reach the irradiation region allows droplet 608 a to establish an EUV emitting plasma upon irradiation by the laser 22′ while droplet 608 b shields subsequent droplet pair 610 from plasma debris.

FIG. 8 illustrates a droplet pattern 700 achievable using a modulated e.g., multiple frequency, disturbance waveform in which droplet pairs reach the irradiation region with a first droplet 702 a reflecting light into a self-directing laser system 704 to initiate a laser oscillation output laser beam which irradiates the second droplet 702 b to produce an EUV emitting plasma.

Self-directing laser system 704 is more fully described in co-pending U.S. patent application Ser. No. 11/580,414, filed on Oct. 13, 2006, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0025-01. See, in particular, FIG. 5 of U.S. Ser. No. 11/580,414, the entire contents of which were previously incorporated by reference. Although the following describes a laser system 704 corresponding to FIG. 5 of the patent application Ser. No. 11/580,414 , it is to be appreciated that this description is equally applicable to the other self-directed lasers disclosed in the patent application Ser. No. 11/580,414 (i.e., FIGS. 6-16.)

Continuing with FIG. 8, it can be seen that the self-directing laser system 704 may include an optical amplifier 706 a,b,c. For example, the optical amplifier 706 may be a CW pumped, multiple chamber, CO₂ laser amplifier amplifying light at a wavelength of 10.6 μm and having a relatively high two-pass gain (e.g., a two pass gain of about 1,000,000). As further shown, the amplifier 706 may include a chain of amplifier chambers 706 a-c, arranged in series, each chamber having its own active media and excitation source, e.g., electrodes.

In use, the first droplet 702 a of target material is placed on a trajectory passing through or near a beam path 710 extending through the amplifier 706. Spontaneously emitted photons from the amplifier 706, may be scattered by the droplet, and some scattered photons may be placed on path 710 where they travel though the amplifier 706. As shown, an optic 708 may be positioned to receive the photons on path 710 from the amplifier 706 and direct the beam back through the amplifier 706 for subsequent interaction with the second droplet 702 b to produce an EUV light emitting plasma. For this arrangement, the optic 708 may be, for example, a flat mirror, curved mirror, phase-conjugate mirror or corner reflector. An optical element 714, e.g., lens may be positioned to collimate light entering the amplifier 706 from the droplet and focus light traveling from the amplifier 706 to the droplet. An optional optical delay 716 may be provided to establish the required time delay between when the first and second droplets reach the irradiation region. One advantage of using different droplets is that the size of the droplets may be independently optimized for their specific function (i.e., reflection versus plasma production).

FIG. 8A illustrates a droplet pattern 700′ achievable using a modulated e.g., multiple frequency, disturbance waveform (described above), or a pulsed waveform (described below), in which droplet doublets reach the irradiation site with each droplet in the droplet doublet being irradiated to produce plasma. Specifically, both droplets in the doublet produce plasma and EUV light, and both droplets may be irradiated by a laser beam produced with a single gain inversion in the amplifiers 706 a′-c′. For this arrangement, an increased EUV output power may be obtained for each gain inversion relative to the EUV output produced with a single droplet. In some cases, it may be advantageous to use droplets, such that at least one droplet in the droplet doublet has a diameter, d, approaching the irradiation site, and center-to-center droplet spacing within each doublet, D, with d≦D≦4d. The droplets may be substantially equal in diameter, or one droplet may be larger than the other.

Continuing with FIG. 8A, it can be seen that the self directing laser system 704′ may include optical amplifiers 706 a′,b′,c′. For example, the optical amplifier 706′ may be a CW pumped, multiple chamber, CO₂ laser amplifier amplifying light at a wavelength of 10.6 μm and having a relatively high two-pass gain (e.g., a two-pass gain of about 1,000,000). As further shown, the amplifier 706′ may include a chain of amplifier chambers 706 a′-e′, arranged in series, each chamber having its own active media and excitation source, e.g., electrodes.

In use, the first droplet 702 b′ of target material is placed on a trajectory passing through or near a beam path 710′ extending through the amplifier 706′. Spontaneously emitted photons from the amplifier 706′ may be scattered by the droplet, and some scattered photons may be placed on path 710′ where they travel though the amplifier 706′. As shown, an optic 708′ may be positioned to receive the photons on path 710′ from the amplifier 706′ and direct the beam back through the amplifier 706′. A laser beam may then be established along beam path 710′ irradiating droplet 702 b′ and producing and EUV light emitting plasma and continue circulating in the optical cavity established between the plasma and optic 708′ until droplet 702 a′ reaches beam path 710′. Droplet 702 a′ is then irradiated to produce EUV light emitting plasma. For this arrangement, the optic 708′ may be, for example, a flat mirror, curved mirror, phase-conjugate mirror or corner reflector. An optical element 714′, e.g., lens, may be positioned to collimate light entering the amplifier 706′ from the droplet and focus light traveling from the amplifier 706′ to the droplet. In some implementations, the droplets in the doublet may coalesce forming an elongated droplet prior to, or during, irradiation.

With reference now to FIGS. 9-12, Applicants have determined that in addition to the modulated, e.g., multiple frequency, disturbance waveforms described above, other waveforms may be used to produce coalescing droplet streams that can be controlled to produce a stable stream of coalesced droplets below the frequency minimum (about 0.3 υ/(πd) as described above), that would otherwise limit stable droplet production using single frequency sinusoidal non-modulated waveform disturbances.

Specifically, these waveforms may produce a disturbance in the fluid which generates a stream of droplets having differing initial velocities within the stream that are controlled, predicable, repeatable and/or nonrandom.

For example, for a droplet generator producing a disturbance using an electro-actuable element, a series of pulse waveforms may be used with each pulse having sufficiently short rise-time and/or fall-time compared to the length of the waveform period to generate a fundamental frequency within an operable response range of the electro-actuatable element, and at least one harmonic of the fundamental frequency.

As used herein, the term fundamental frequency, and its derivatives and equivalents, means a frequency disturbing a fluid flowing to an outlet orifice and/or a frequency applied to a sub-system generating droplets, such as a nozzle, having an electro-actuatable element producing a disturbance in the fluid; to produce a stream of droplets, such that if the droplets in the stream are allowed to fully coalesce into a pattern of equally spaced droplets, there would be one fully coalesced droplet per period of the fundamental frequency.

Examples of suitable pulse waveforms include, but are not necessarily limited to, a square wave (FIG. 9), rectangular wave, and peaked-nonsinusoidal waves having sufficiently short rise-time and/or fall-time such as a fast pulse (FIG. 13A), fast ramp wave (FIG. 14A) and a sine function wave (FIG. 15A).

FIG. 9 shows a representation of a square wave 800 as a superposition of odd harmonics of a sine wave signal. Note: only the first two harmonics of the frequency f are shown for simplicity. It is to be appreciated that an exact square wave shape would be obtained with an infinite number of odd harmonics with progressively smaller amplitudes. In more detail, a square wave 800 can be mathematically represented as a combination of sine waves with fundamental frequency, f, (waveform 802) of the square wave and its higher order odd harmonics, 3 f, (waveform 804), 5 f (waveform 806); and so on:

${v(t)} = {\frac{4}{\pi}\left( {{\sin \left( {\omega \; t} \right)} + {\frac{1}{3}{\sin \left( {3\omega \; t} \right)}} + {\frac{1}{5}{\sin \left( {5\omega \; t} \right)}} + {\frac{1}{7}{\sin \left( {7\omega \; t} \right)}} + \ldots} \right)}$

where t is time, v(t) is the instantaneous amplitude of the wave (i.e. voltage), and ω is the angular frequency. Thus, applying a square wave signal to an electro-actuatable element, e.g., piezoelectric, may result in mechanical vibrations at the fundamental frequency f=ω/2π, as well as higher harmonics of this frequency 3 f, 5 f, etc. This is possible due to the limited and, in general case, highly nonuniform frequency response of a droplet generator employing an electro-actuatable element. If the fundamental frequency of the square wave signal significantly exceeds the limiting value of 0.3 υ(πd), then the formation of single droplets at this frequency is effectively prohibited and the droplets are generated at the higher harmonics. As in the case of the amplitude and frequency modulation described above, droplets produced with a square wave signal have differential velocities, relative to adjacent droplets in the stream, that lead to their eventual coalescence into larger droplets with a frequency f. In some implementations, the EUV light source is configured such that a plurality of droplets are produced per period, with each droplet having a different initial velocity than a subsequent droplet, such that: 1) at least two droplets coalesce before reaching the irradiation site; or 2) the droplets produce a desired pattern such as a pattern which includes closely-spaced, droplet doublets (see discussion below).

FIGS. 10 and 11 show images of droplets obtained with a square wave modulation at 30 kHz. With a simple sine wave modulation, the lowest modulation frequency where a single droplet per period can be obtained for the droplet generator used in this experiment was 110 kHz. The image shown in FIG. 10 was taken at ˜40 mm from the output orifice and the image shown in FIG. 11 was taken later at ˜120 mm from the output orifice where the droplets are already coalesced. This example demonstrates the advantage of using a square wave modulation to obtain droplets at a frequency lower than the natural, low-frequency limit of a particular droplet generator configuration.

Similar arguments can be applied to a variety of repetitive modulation signals with multiple harmonics having short rise-time and/or fall-time including, but not limited to, a fast pulse (FIG. 13A), fast ramp wave (FIG. 14A) and a sine function wave (FIG. 15A). For instance, a saw tooth waveform contains not only odd, but also even harmonics of the fundamental frequency and therefore can also be effectively used for overcoming the low frequency modulation limit and improving stability of a droplet generator. In some cases, a specific droplet generator configuration may be more responsive to some frequencies than others. In this case, a waveform which generates a large number of frequencies is more likely to include a frequency which matches the response frequency of the particular droplet generator.

FIG. 12A shows a rectangular wave 900 for driving a droplet generator and FIG. 12B shows a corresponding frequency spectrum having fundamental frequency 902 a and harmonics 902 b-h of various magnitudes for a period of the rectangular wave. FIG. 12C shows an image of droplets taken at 20 mm from the output orifice of the droplet generator driven by the rectangular wave and shows droplets beginning to coalesce. FIG. 12D shows an image of droplets taken at 450 mm from the output orifice after the droplets have fully coalesced.

FIG. 13A shows a series of fast pulses 1000 for driving a droplet generator and FIG. 13B shows a corresponding frequency spectrum having fundamental frequency 1002 a and harmonics 1002 b-i of various magnitudes for a period of the rectangular wave. FIG. 13C shows an image of droplets taken at 20 mm from the output orifice of the droplet generator driven by the series of fast pulses and shows droplets beginning to coalesce. FIG. 13D shows an image of droplets taken at 450 mm from the output orifice after the droplets have fully coalesced.

FIG. 14A shows a fast ramp wave 1100 for driving a droplet generator and FIG. 14B shows a corresponding frequency spectrum having fundamental frequency 1102 a and harmonics 1102 b-p of various magnitudes for a period of the rectangular wave. FIG. 14C shows an image of droplets taken at 20 mm from the output orifice of the droplet generator driven by the fast ramp wave and shows droplets beginning to coalesce. FIG. 14D shows an image of droplets taken at 450 mm from the output orifice after the droplets have fully coalesced.

FIG. 15A shows a sine function wave 1200 for driving a droplet generator and FIG. 15B shows a corresponding frequency spectrum having fundamental frequency 1202 a and harmonics 1202 b-1 of various magnitudes for a period of the rectangular wave. FIG. 15C shows an image of droplets taken at 20 mm from the output orifice of the droplet generator driven by the sine function wave and shows droplets beginning to coalesce. FIG. 15D shows an image of droplets taken at 450 mm from the output orifice after the droplets have fully coalesced.

While the particular embodiment(s) described and illustrated in this Patent Application in the detail required to satisfy 35 U.S.C. §112 are fully capable of attaining one or more of the above-described purposes for, problems to be solved by, or any other reasons for, or objects of the embodiment(s) above-described, it is to be understood by those skilled in the art that the above-described embodiment(s) are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the present application. Reference to an element in the following Claims in the singular, is not intended to mean nor shall it mean in interpreting such Claim element “one and only one” unless explicitly so stated, but rather “one or more”. All structural and functional equivalents to any of the elements of the above-described embodiment(s) that are known, or later come to be known to those of ordinary skill in the art, are expressly incorporated herein by reference and are intended to be encompassed by the present Claims. Any term used in the Specification and/or in the Claims and expressly given a meaning in the Specification and/or Claims in the present Application shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term, It is not intended or necessary for a device or method discussed in the Specification as an embodiment to address or solve each and every problem discussed in this Application for it to be encompassed by the present Claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the Claims. No claim element hi the appended Claims is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited as a “step” instead of an “act”. 

1. A device comprising: a plasma generating system comprising a source of target material droplets and a laser producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation, wherein the droplet source comprises a fluid exiting an orifice and a sub-system producing a disturbance in the fluid which generates droplets having differing initial velocities causing the spacing between at least some adjacent droplets to decrease as the droplets travel to the irradiation region.
 2. A device as recited in claim 1 wherein at least some adjacent droplets coalesce prior to reaching the irradiation region.
 3. A device as recited in claim 1 wherein said sub-system produces a droplet pattern wherein droplet doublets reach the irradiation site with each droplet in the droplet doublet being irradiated to produce plasma.
 4. A device as recited in claim 3 wherein at least one droplet in the droplet doublet has a diameter, d, approaching the irradiation site, and center-to-center droplet spacing within each doublet, D, with d≦D≦4d.
 5. A device as recited in claim 1 wherein the disturbance comprises a frequency modulated disturbance waveform.
 6. A device as recited in claim 1 wherein the disturbance comprises an amplitude modulated disturbance waveform.
 7. A device as recited in claim 1 wherein the sub-system produces a series of pulsed disturbances, with each pulsed disturbance having at least one of a sufficiently short rise-time and sufficiently short fall-time to generate a fundamental frequency and at least one harmonic of the fundamental frequency.
 8. A device comprising: a plasma generating system comprising a source of target material droplets and a laser producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation, wherein the droplet source comprises a fluid exiting an orifice and a sub-system having an electro-actuatable element producing a disturbance in the fluid, the electro-actuatable element driven by a waveform having at least one of a sufficiently short rise-time and a sufficiently short fall-time to generate a fundamental frequency within an operable response range of the electro-actuatable element and at least one harmonic of the fundamental frequency.
 9. A device as recited in claim 8 wherein the waveform is selected from the group of waveforms consisting of a square wave, rectangular wave and peaked-non-sinusoidal wave.
 10. A device as recited in claim 8 wherein the peaked-non-sinusoidal wave is selected from the group of waveforms consisting of a fast pulse waveform, a fast ramp waveform and a sine function waveform.
 10. A device as recited in claim 8 wherein the at least one harmonic is an odd harmonic of the fundamental frequency.
 11. A device as recited in claim 8 wherein the at least one harmonic comprises at least one even harmonic of the fundamental frequency and at least one odd harmonic of the fundamental frequency.
 12. A device as recited in claim 8 wherein the waveform generates droplets having differing initial velocities causing the spacing between at least some adjacent droplets to decrease as the droplets travel to the irradiation region.
 13. A device as recited in claim 12 wherein at least some adjacent droplets coalesce prior to reaching the irradiation region.
 14. A device as recited in claim 8 wherein each pulse is generated by changing an on-off switch state.
 15. A method comprising the steps of; flowing a fluid toward an orifice; disturbing the fluid to generate a first droplet and a second droplet, the first droplet having a different initial velocity than the second droplet causing the spacing between the first and second droplets to decrease as the droplets travel to an irradiation region; and irradiating droplet material at the irradiation region to form a plasma.
 16. A method as recited in claim 15 wherein the first and second droplets coalesce prior to reaching the irradiation region.
 17. A method as recited in claim 15 wherein the disturbance comprises a waveform selected from the group of modulated waveforms consisting of a frequency modulated disturbance waveform and an amplitude modulated disturbance waveform.
 18. A method as recited in claim 15 wherein the step of disturbing the fluid comprises producing a series of pulsed disturbances, with each pulsed disturbance having at least on of a sufficiently short rise-time and a sufficiently short fall-time to generate a fundamental frequency and at least one sub-harmonic of the fundamental frequency.
 19. A method as recited in claim 18 wherein the pulsed disturbance is selected from the group of disturbance waveforms consisting of a square wave, rectangular wave, and peaked non-sinusoidal wave. 